TDM telephone systems operate on a time division basis, that is a number of conversations share a single communication path. Conversations are each assigned to the communication path for short periodically recurring intervals. Samples of the conversation are coded into corresponding binary words using pulse code modulation techniques. These words are transmitted over the communication path during assigned time slots and are decoded into the original conversations, in receiving terminals. A TDM switching system usually has two outgoing paths and two incoming paths connectd between a network in the switch system and the associated user communication terminals. A conversation between two user terminals is allocated one time slot and is transmitted via the incoming and outgoing paths. Both of the incoming and outgoing paths are used to maintain each side of the conversation separate from the other. The network determines the incoming paths and outgoing paths of the two user terminals and the signal on the incoming path of one is transferred to the outgoing path of the other. The user terminals are enabled under the control of an interconnection memory in the switching system. The memory includes a word location corresponding to each time slot in a time slot frame. The memory word at each location contains the address of the two terminals to be connected together, as well as the identity of the paths which need to be enabled to achieve the interconnection.
When the capacity of such a system is expanded, additional pairs of incoming and outgoing paths are needed and space and time switching is required to enable the user terminals associated with the different pairs of paths to converse. This requires a corresponding expansion in memory capacity. In addition, an expansion of the length of the memory words themselves becomes necessary. Each word must now contain time slot information to enable a data memory to transfer samples between time slots and space switch information to enable a space switch to interconnect various of the incoming and outgoing paths are required. Hence, the increased cost of memory capacity is usually disproportionately large relative to the increase in the number of user terminals.
One example of a time division switching system is taught by M. J. Marcus in U.S. Pat. No. 3,573,381 issued on Apr. 6, 1971 and entitled "Time Division Switching System". This system has a plurality of combination space switching and data storage devices which transpose data between various time channels during its transmission between multichannel, time multiplex highways. Although the system is quite flexible and has a high traffic capability, it requires a substantial amount of memory, both for controlling the operation of the switching network and for storing the data being transmitted.
Another example of a time division switching system is described in U.S. Pat. No. 3,694,580, issued on Sept. 26, 1972 to Hiroshi Inose and Tadao Saito and entitled "Time Division Switching System". This system switches individual information bits of pulse code modulation words between various time slots and time division buses. FIGS. 4A and 4B depict pulse shifters which each include two shift registers and a gating matrix connected between the shift registers. The gating matrix is controlled by memory to select a new sequence of the information bits in each frame. This system is relatively flexible and has a high traffic capability; however, a substantial amount of memory is required for the control of the pulse shifters.
Prior TDM PCM switching systems in general require large amounts of memory and provide high traffic handling capability. In the situation where only a few hundred telephones and relatively low traffic requirements exist, the known PCM TDM systems are too expensive to provide a practical alternative to other forms of TDM systems, such as pulse amplitude modulation (PAM) or delta modulation systems. On the other hand, telephone systems tend to expand with the passage of time. Because of their cost advantage, PAM or delta modulation systems are acceptble in a small system in spite of their well-known disadvantages. However, they are not acceptable in larger systems where the their cost advantage is insignificant or non-existent.
The private branch exchange (PBX) user is presently faced with the choice of initially installing a prohibitively expensive TDM PCM system which is much larger than his present requirement, or a much less expensive system which is expensive to enlarge or cannot be expanded. More often than not, the less expensive system which meets his present requirements but cannot economically meet his future requirements is chosen. However, beyond a certain system size, expansion costs increase rapidly. At some time a new and larger system must be substituted for the old system. Of course, the purchasing and installing of a new system results in substantial expenditures in addition to the cost and inconvenience of removing and disposing of the old system.